The objectives are to provide a better understanding of the anatomy, physiology and biochemistry of the gastric mucosal barrier and to elucidate those factors which influence the ability of the mucosa to protect itself against ulceration. A new conceptualization of the barrier as a dynamic phenomenon dependent upon many functional variables has recently been evolved in our laboratory replacing the previous older formulation of a fixed anatomical or physiological gate. The capacity of the mucosa to cope with H ion will be examined in relation to the functional and acid-base status of the mucosa. An attempt will be made to separate the function of the surface and oxyntic cells and the role of CO2 and carbonic anhydrase will also be evaluated. Similar aspects of duodenal disposal of H ion will be examined. The basic in-vitro preparation will consist of isolated bullfrog antrum or fundus in an Ussing chamber. In addition to the usual electrical measurements, CO2 production will be determined. Attempts will be made to measure intracellular and intramucosal pH. In vivo, a specially prepared flap of canine or rabbit antral or fundic mucosa to which blood flow and blood pressure can be varied will be used.